Does the speaker understand the scene on the urn?
The narrator here is the poet himself (John Keats), not so much conjecturing on the urn’s artist’s intentions, or trying to tell the story illustrated on the urn, as giving the silent figures a more modern romantic interpretation, in the Romantic tradition of finding natural, universal themes in human activities. His description, expressed in the interrogative mode, shows that he is “asking questions” but not pretending to interpret the actual scene. Whether Keats actually had knowledge of the Greek legend or action depicted on the urn is not open to question, since he does not address its historical provenance as a Greek artifact (although his biography shows he was well educated in the classics), any more than Wordsworth’s mention of a nightingale in “The Solitary Reaper” implies a knowledge of bird physiology. The Ode is “on” a Grecian Urn, meaning “ponderings on the random thoughts brought to mind while gazing on the urn”. One could write an ode “on” what thoughts came to mind when gazing “on” a burial urn, without ever discussing what the urn’s decorations “meant.